This invention generally relates to a radio data transmission system and more particularly to a mobile or portable radiotelephone system having the capacity of efficiently transmitting data over a duplex radio channel which may change in frequency as the remote subscriber unit changes location.
Mobile radiotelephone service has been in use for some time and traditionally has been characterized by a central site transmitting with high power to a limited number of mobile units in a large geographic area. Mobile transmissions, due to their lower power, are received by a network of receivers located remote from the central site and returned to the central site. Because of a limited number of radio channels available (23 channels but, because of interference considerations, only 15 channels are effectively available), a maximum number of conversations for an entire city would equal the few channels available. Consequently, mobile telephone users discovered that radiotelephone was different than landline telephone due to the often busy conditions of the channels.
The reallocation of the 800 to 900 MHz spectrum in the early 1970's has made available approximately 1000 channels of mobile radiotelephone use in a band of frequencies well suited for limited range propagation. Cellular technology developed in response to the need and the reallocation.
A cellular system characteristically has the coverage area divided into contiguous smaller coverage areas (cells) using low power transmitters and receivers. One cellular system is further described in U.S. Pat. No. 3,906,166 assigned to the assignee of the present invention. The limited coverage area enables the channel frequencies used in one cell to be reused in another cell geographically separated according to a predetermined plan. One such plan is disclosed in U.S. Pat. No. 4,128,740, Graziano, assigned to the assignee of the present invention. Thus a large number of channels can be made available in a metropolitan area and the service can appear to be identical to a standard telephone.
The cell system typically utilizes one channel in each cell to receive requests for service (on a "reverse set-up" frequency) from subscriber units, to call subscriber units (on a "forward set-up frequency" and to instruct subscriber units to tune to a frequency pair where a conversation may take place (a "voice" channel). The one "set up" channel in each cell is continuously assigned the task of receiving and transmitting data and is the channel to which the subscriber unit tunes when not in a conversational state.
Since the cells may be of relatively small size (approximately 10 miles in radius), the likelihood of a mobile or portable subscriber unit traveling out of one cell and into another is high. To maintain communications, the subscriber unit is "handed-off" between one cell and another. The cell systems in use track the unit and decide when a handoff is necessary to maintain quality communications. The subscriber unit is commanded, via a high speed data message interrupting the audio communications on the voice channel, to retune the transceiver to another frequency which is available in a new cell. This handoff requires a relatively short period of time and the user is generally unaware of the occurrence.
The data messages are transmitted in a Manchester encoded format, a format which is well known by those skilled in the art, at a speed of 10 kilobits per second in the United States. (Other countries such as the United Kingdom, have different data speeds such as 8 kilobits per second). This digital transmission system has been thoroughly studied for application to high capacity mobile telephone systems and the error rates. spectrum occupancy. Immunity to radio channel fading, and other performance characteristics have been extensively reported.
Since cellular telephone systems provide performance characteristic of the land line telephone system and interconnect with it, subscribers expect land telephone system features from the cellular telephone system. One such feature is the transmission of data from one location to another. Many telephone subscribers connect data communications devices, such as a personal computer, to the telephone system via a modem. Modems are familiar to those skilled in the art and fundamentally operate by converting data "1" and ".0." levels to distinct tones or to particular tone waveform phase relationships which can be transmitted by the land telephone network.
It would be natural to connect a computing device via a modem to a radiotelephone subscriber unit for communication with another data generating device via the land telephone network. In fact, this has been done and produced unsatisfactory results. Rapid multipath fading, commonly experienced in the high frequency cellular radiotelephone communications, causes gaps and significant phase changes in modem-generated tones such that data carried by the radio channel becomes garbled or missing. Furthermore, a handoff between cells, which to a human engaged in conversation is virtually unnoticeable, becomes a formidable obstacle for the communication of data generated by a data generating device.